In recent years, laser light has been used in various applications; for example, such light is used for cutting and working metals, is used as a light source for photolithographic devices in semiconductor manufacturing apparatuses, and is also used in various types of measuring devices, and in procedures and treatment devices used in surgery, ophthalmology, dentistry, and the like. In particular, laser light has recently come to be used in the irradiation of the cornea in order to perform ablation of the surface of the cornea (PRK) or ablation of the interior of the surgically opened cornea (LASIK), and the performance of therapy for nearsightedness, farsightedness and astigmatism by correcting the curvature and irregular indentations and projections of the cornea has attracted attention, and has begun to be adapted for practical use in some cases. Devices which perform ablation of the surface of the cornea by irradiating the cornea with ArF excimer laser light (wavelength: 193 nm) have been known as such corneal treatment devices.
However, ArF excimer laser oscillators are constructed with argon gas, fluorine gas, neon gas or the like sealed inside the chamber, so that tight sealing of these gases is required. Furthermore, filling and recover of the respective gases must also be performed, so that the problem of increased size and complexity of the apparatus is encountered. Moreover, in the case of an ArF excimer laser oscillator, the following problem also arises: namely, periodic replacement of the internal gas or an overhaul of the apparatus is required in order to maintain a predetermined laser light generating performance.
Accordingly, it is desirable to use a solid-state laser instead of such a gas laser as a laser light source. However, the wavelength of the laser light emitted from a solid-state laser is ordinarily longer than the wavelength described above, so that such solid-state lasers are not suited for use in (for example) corneal treatment devices. Therefore, a method has been developed in which long-wavelength light emitted from such a solid-state laser is used after being converted into short-wavelength ultraviolet light (e.g., an eighth wave) by using a nonlinear optical crystal. For example, such a technique is described in Japanese Patent Application Kokai No. 2001-353176. LBO crystals, SBBO crystals, and the like are known as nonlinear optical elements that are used for such purposes.
In such wavelength conversion optical systems, the laser light from the laser light source constituting the fundamental wave is focused using a focusing lens, and is caused to be incident on a nonlinear optical element. In cases where the phase matching conditions are satisfied, the intensity of the laser light that is produced by wavelength conversion is proportional to the square of the intensity of the fundamental wave; accordingly, it is particularly essential that the intensity of the output light be increased by using a focusing lens to focus the light. The laser light produced by wavelength conversion using a nonlinear optical element is shaped into a desired beam shape by means of a lens in accordance with the intended use.
Meanwhile, fuzed silica glass is widely used as a material for lenses used in the ultraviolet region for reasons such as the fact that such glass has a superior transmissivity in the ultraviolet region, and the thermal expansion coefficient of such glass is extremely small, so that the glass has superior temperature stability.
Fuzed silica glass has such superior characteristics as those described above in the ultraviolet region; however, it has been ascertained that such glass is damaged by irradiation with ultraviolet laser light, for example, at a wavelength of 248 nm or 193 nm.
Accordingly, the following problem has been encountered: namely, lenses made of a fuzed silica glass must be replaced at predetermined intervals of use. As a countermeasure in this case, it is conceivable that the useful life might be extended by shifting the lens so that unused portions are newly used, thus avoiding portions that have been subjected to damage. In this case, however, the following problem arises: namely, the optical axis is shifted.
The present invention was devised in light of such circumstances; it is an object of the present invention to provide an optical element and optical system that can be used over a long period of time even in cases where this element and system are formed from a material whose performance is caused to be damaged by the light that is used, and also to provide a laser device, exposure apparatus, mask inspection device and macromolecular crystal working apparatus using this optical element and optical system.